D-galactose is a monosaccharide, which is mostly found in the milk sugar or lactose, where galactose is bound to d-glucose. D-galactose has the following structure:

In some sour milk products, lactose has been decomposed into glucose and galactose. D-galactose has many applications in the pharmaceutical field and in food technology. In the pharmaceutical field, galactose is useful for example as a pharmaceutical intermediate for several medicines. Furthermore, d-galactose is also useful as a stabilizer in intravenous solutions for medical use. In food technology, d-galactose has been found useful for example as a potential energy source in sports drinks. D-galactose is also useful in cell culture media as a nutrient or as an inducer in fermentation processes. D-galactose is typically obtained by hydrolyzing lactose (a disaccharide consisting of glucose and galactose), which is found in dairy products, such as milk.
It is known in the art to recover d-galactose from various plant-based raw materials using methods selected, for example, from extraction, hydrolysis and treatment with adsorbents and cation and anion exchangers, followed by crystallization. Chromatographic methods for the recovery of d-galactose-containing solutions from plant-based materials are also known in the art.
D-tagatose is a ketohexose having the following structure:

D-tagatose may be formed from d-galactose by enzymatic isomerization. Typically, the isomerization is carried out in the presence of L-arabinose isomerase under alkaline conditions in the presence of calcium. D-tagatose is useful as a food additive, as a sweetener, as a texturizer, as a stabilizer, or as a humectant. D-tagatose is also useful in formulating dietic foods with a low glycemic index. Potential applications of d-tagatose include breakfast cereals, diet soft drinks, reduced fat ice cream, hard and soft candies, chewing gums, dietary supplements, and special diet food for meal replacement.
D-tagatose is typically produced in a two-step process wherein lactose is enzymatically hydrolyzed to d-Galactose and d-glucose using immobilized lactase. The d-galactose is typically separated using a cation exchange resin. The separated d-galactose is then isomerized to produce d-tagatose under alkaline conditions (typically at a pH of 12) using calcium hydroxide to form a precipitate. The precipitate is subsequently treated with sulfuric acid to free the d-tagatose, and the filtrate is demineralized in a cation and anion exchanger. Typically, the resulting solution is concentrated and purified by chromatic fractionation using a cation exchanger. The d-tagatose is recovered by crystallization.
Simulation of a moving sorbent bed is described in U.S. Pat. No. 2,985,589 (Broughton et al.), which is mentioned above. In accomplishing this simulation, it is necessary to connect a feed stream to a series of beds in sequence, first to bed no. 1, then to bed no. 2, and so forth for numerous beds, the number of beds often being between 12 and 24. These beds may be considered to be portions of a single large bed whose movement is simulated. Each time the feed stream destination is changed, it is also necessary to change the destinations (or origins) of at least three other streams, which may be streams entering the beds, such as the feed stream, or leaving the beds. The moving bed simulation may be imply described as dividing the bed into series of fixed beds and moving the points of introducing and withdrawing liquid streams past the series of fixed beds instead of moving the beds past the introduction and withdrawal points. A rotary valve used in the Broughton process may be described as accomplishing the simultaneous interconnection of two separate groups of conduits.
U.S. Pat. No. 4,412,866 describes an example of the operation of chromatographic simulated moving bed (or sometimes called “SMB”) method to separate the components of a feed stock. A resin bed is divided into a series of discrete vessels, each of which functions as a zone within a circulation loop. A manifold system connects the vessels and directs, in appropriate sequence to (or from) each vessel, each of the four media accommodated by the process. Those media are generally referred to as feed stock, eluent, extract and raffinate, respectively. As applied to a sugar factory, a typical feed stock is a lower purity sucrose solution, the eluent is water, the extract is an aqueous solution of sucrose and the raffinate is an aqueous solution containing non-sucrose, such as salts and high molecular weight compounds. The simulated moving bed disclosed by the '866 patent is of the type sometimes referred to as a “continuous SMB.”
An example of a batch chromatographic method for the purification of sucrose is described in the disclosure of U.S. Pat. No. 4,359,430, which utilizes sucrose feedstocks derived from sugar beets at purities of approximately 7% to 60% sucrose. See also, e.g., U.S. Pat. No. 5,466,294, which utilizes a “soft raw syrup” as a feedstock to a chromatographic method which is not in a high purity form at a less than 89% purity sucrose on a dry solids basis, i.e., approximately 11% non-sucrose impurities.
U.S. Pat. No. 6,057,135 discloses a method of producing d-tagatose from lactose hydrolysate, comprising glucose and d-galactose. The method comprises subjecting the lactose hydrolysate to fermentation conditions whereby the glucose is selectively fermented to ethanol. The remaining d-galactose is separated from the ethanol to provide a solution having a concentration of from about 10% to about 60% by weight d-galactose. The solution of d-galactose is subjected to enzymatic isomerization with L-arabinose isomerase at an isomerization pH from about 5.5 to about 7.0 and a temperature from about 50° C. to about 70° C. The resulting yield of d-tagatose is from about 20% to about 45% by weight based on d-galactose.
U.S. Pat. No. 7,931,751 discloses a method for purifying already high purity sucrose using a simulated moving bed chromatographic wherein a strong acid cation resin is employed as the stationary phase and water is used as the mobile phase desorbent or chromatographic eluent. The method is disclosed to separate the relatively small qualities of non-sucrose impurities and produce a waste stream which is sufficiently low in solids that it can be sent directly to water disposal facilities with little or no concentration required.
Methods are sought for the separation of a mixture of sugars where more than one of the sugars can be produced as a product stream.
Methods are sought for a more efficient method of producing d-tagatose from lactose hydrolysate.